Thermionic discharge device



Oct. 11, 1949. F. E, GEHRKE 2,484,703

THERMIONIC DIS CHARGE DEVICE Filed July l, 1948 --illlllllll-- Y 25u/cf fla 5 INI/ENTOR.

Patented Oct. 1l, 1949 THERMIONIC DISCHARGE DEVICE 'Forrest E. Gehrke, Flushing, N.'Y., assignor to Sylvania Electric .Products Inc., a corporation of Massachusetts Application July 1, 1948serial No. 36,333

The present invention relates to thermionic discharge devices and more particularly to gas discharge devices depending, for their operation, upon ionization of the contained gas.

An object of the present invention is the improvement of the construction and operation of grid controlled gas discharge devices.

Another object of the present invention is the provision of a simpliied shield grid assembly for a shield grid gas discharge tube.

A further object of the present invention is the provision of forms of control grid and shield grid construction for use in grid controlled gas discharge tubes such that they may be stamped out of strip material as a nished part and thus not require further assembling operations.

Still another object of the present invention lis the provisionof a grid controlled gas discharge tube in which large variations in dimensions of the parts of the tube have little effect on the operating characteristics of the tube.

Still a, further object ofthe present invention is the provision of a grid controlled gas discharge tube which is tolerant of misalignment of parts during assembly.

Still a further'object of the present invention is the construction of a grid controlled gas discharge tube which isuniform in its characteristics and which maintains these characteristics throughout its life.

Still another object of the present invention is the provision of 'a grid controlled gas discharge tube in which the eiect of cathode contamination is minimized.

The foregoing objects, and others which may appear from the following detailed description, are attained by providing within an hermetically sealed casing containing an ionizable gas, a tetrode assemblyincluding a heated cathode, a control grid, a shield grid, and an anode. The shield grid and control grid are so designed that they can be made'as 'unitary parts of strip ma terial on high speed stamping and forming machinery. 'v

The arrangement of the control grid surfaces with respect to the shield grid assembly is such as to prevent crowding of equipotential lines in the space between the anode and cathode. Furthermore, the control grid is so arranged that the surfaces of the grid are hidden from the cathode and from the plate, thus providing a very low grid-to-anode. capacitance and grid-tocathode capacitance. l

In an embodiment of the present invention the shield grid is preferablyfiormed generally 11 claims. (c1. 25o-27.5)

-anodev I6, and a shield grid or shield I8.

in somewhat the shape of an 'hour glass or a lemniscate having a constricted passage between its loops. Within the central neck portion of the shield grid assembly is arranged the control grid, which is in the form of a flat loop of strip material. `The cathode is arranged in one of the enlarged end spaces of the shield grid and the anode in the other, i

'The present invention will be more fully understood by reference to the following detailed description, which is accompanied by a drawing in which:

Fig. 1 illustrates, in elevation and partly in section; a tube embodying principles of the present invention, while Fig. 2 isa transverse cross-section of the tube shown in Fig. 1 taken along the lines 2-2 of Fig. 1;

FigjB is a plot of the electrostatic field withinv thev tube of Figs. l and 2, while Fig. 4 is a Asimilar plot of the electrostatic eld in certain prior art tubes, with which I amY familiar, and f Fig. 5 is a schematic circuit diagram showing the relationships of the various elements within the tubeof the present invention and their consequent inter-element capacity effects.

In Fig. 1 isshown a thermionic discharge tube having an hermetically sealed glass envelope I0 containing an ionizable gas such as argon, mercury vapor or helium. (Other gases may be used if desired.) Within the tube envelope is arranged a tetrode thermionic discharge assembly including a cathode I2, a control grid I4, an The tetrode parts are maintained in their relative position within the glass envelope. I0 by being secured to mica spacers I 9 and 20. I

1- In a preferredform of construction, the cathode in the vform of a hollow tube having a heater element, not shown, therewithin, and an electron emissive coating on the exterior, is maintained in position by being snugly tted through holes in lthe mica spacers I9 and 20. The grid I4 is formed of flat sheet stock bent over on itself and slightly spread apart in its `intermediate portions to form an open loop in the region between spacers I9 and 2D. The end portions 23 and 24 are vflattened together to pass through flat slots in the mica. An'ode I6 is preferably secured in place by curved ears 25 and '26 extending throughv arcuately shaped apertures in the spacers I9 and 20. Shield gridy IB maybe similarly secured in place, or, as shown in the present drawing, support rods 28` passing through the mica end shield grid I8 is generally of hour. glass forma-v.

tion having the control grid I4 in the neck portion of the hour glass, andrthe:cathode`Y land.

anode I6 each in one of theeenlargedportions of the shield grid.

In order to facilitate processing thegtube; ,itzfisz desirable to provide a narrow window or aperture el in the shield grid directly behind theanedeglsi.

Anode I6, however, entirely covers the window so no strayY electron streamsffromrthefcathode are able -to emerge through` theywindowf 3 I,

InFig. 3 Lhave shown. schematically, the same general arrangementof. electrodes a'seshownvin the cross-section in Fig. 2, but/'have addedia ,number of dotted,v lines .40.7 indicating: equipotential Alines between the` cathode I2,andthe anod'eII. `It willy be noted in Fig. 3 that the ,equipotential'lineslli are not crowdedy atanyl point, but rather follow smoothly flowing contourss., Crowding'gof the equi;- potential lines indicateshigh v,petential gradients. Since it is the potential gradients along angelectron streamthat effect.thef.direction and-velocity of. the electrons` in; therstream;v it-,may,be seen that the slightest variation in dirnensionsgzofv the opening of a. grid,- having a point-,0r thin line as the active elementmay have very larg-e1` effectsupon the tubes performance and upon uniformity' of characteristics .between different: tubeszof` the same type. In the-presentarrangementl the ,rev-` verse is true, thus, largevariationsin spacingfand dimensioning may-foccur Without seriously',V eiect ing the potential gradients through which; anyv electron. passes at a rgivengpointl. Thisesituation may becontrastedgwith the;` arrangement shown".

in v Fig. 4 wherein the ,equipkotentiaL lines; aref( in` dicated in a priorg knownforin of. shieldl grid thy-f,

ratron. l-Iere platesil). and 5I constitute apart of a-vshield. grid assembly. n Theymaylv-,be associated j with a surrounding.boxelikescage. It shouldfbe.

carefully noted that` they eduipotentiallines-'52 in Fig.- 4 are extremely, cramped-fand close toi-- gether. where...they-pass Athrough the .aperturesiin platesv-liand 5I.. Thus, veryslight.variations1in dimensions; on in positioningvof; the elementsmay cause. asubstantial:- variation in;l characteristics of tubes constructedinmthismannen Again referring tosl'igt; itfwfill .-be fnoticedtha the control grid I4--isewellhiddenifromf the cathode IZ; thusA keeping the:4 capacity; betweenther control grid and cathodefat-a;minimum. Also, cathode coating,v which may: be. sputteredi oil"V liberated from cathode;y I2: because the; control,`

grid surfaces, i. e?, thegflat.facesaohcontrol grid I4; are hidden fromcathode, I2-ito aggreat extent byy the shieldgrid assembly- 'Ifhus ,-fphotons;liberated.

from -the cathode .-I 2. ,do .not strike lthe *arctigvesurvV faces of the` controL griqzlly Il; topf-cause; secondary;

4 emission. Next, it will be noted, that the control grid I4, or at least its control surfaces, is Well hidden away from the plate I6, thus resulting in a thyratron which has a very low grid to anode capacitance. This is of considerable importance in preventing interaction between voltages appearing at the anode and grid. Ordinarily it is highlyj desirable that thesebe substantially independent'` of"eachf` other, except vaslcoupled by the operating Kcharacteristics of the tube. The way in `wliich capacitive interaction may occur between ,the g rid.. and. cathode is shown in the circuit of Fig; 51' Many `:gas discharge tube applications concerngusesiinwhicliithe grid circuit is placed in an .extremelyihiglriinpedance circuit, of the order of tenrme'gohms'lonhigher, such as that indicated atg. 'IZhisI occurs because it is expected of a gas discharge tube that it will be able to control large amounts of anode current through the application of very minute currents to the grid circuit. It., thee control gridfcathodel capacitance Cgk is high, the reaction:.,time,of theficontiollingecircuit, or,n gridcircuit, is3higl11because the; capacitorY Cgk must be charged. up rfirst through ,the high im,- pedance o f the --gridcircuit.d L

If thecontrol grid.anode.capacity,r Crgp-is high, the relativelylong timefneededfor thehigh impedancein. the gridi circuitf.- toidisc-:harge` this capacitor will prevent: the-icontroll-ing circuit fromr operating correctly, if; ,inif)act,it\y has any control at all. Duefto, the fact-that theycontrol grid; I4

presents only its .'edge .surfaces Ito 1 the fcathode I 2l and theyanode I6:,-,the interelectrode.capacities,

Ggkland Cep; arev small; andgthedeleteri-ous neffects described. abovefareavoidedn Furthermore,` the-shieldv-grid I 8, substantially:-

completely encloses the structureallowingnofree openings; Where the:y electr on.` stream.: from the*v cathode I2, may emerge to impingeupon thegla'sswalls;` TheA effect.I7 ofl-rsuchgimpingementis the'- re'lease of contaminatingvaporsand gases `which adversely aiects.j the operation andVv life.; of the tube.- I

Af Itlvvillbe..y seenthat z-therpresent improved :design:

of a grid Icontrolgas discharge tubeprovides-im-y prov-ed, operating characteristicsfand: a simpler arrangement of,V electrodes?,witliout.` sacrificing' anything. towardsitsimass` producibility and lovv.v

costl.. Furthermore, 1.tests indicate thatgood yuniformity between several tubes ofrthefsame type is obtained, and lifeper-f,ormanceYy tests f have' apneared kto `be substantially{better-than prioriv art` construction even though.A the .Y particular tubes tested were operated with an extremelyflow heater input comparedl ,to thatv ,usedl with.v heretofore available. indirectlyv heatedfcathodelgrid controlled gas-discharge tubes.-` y .e

While I-,haveshown and: particularly described afpreferred;l embodimentof-thegpresentinvention; it. is to,- be.t clearlyf understood-1; that ther-present 2. A grid controlled gas dischargeftub'e-'includ` ing; aq=thermionicifcathode; a control electrodaan anode;` and ayshildf; electrode, said' shield elecscope: of? the" invention may f trode enclosing a pair of enlarged chambers connected by a narrow passage, said cathode being in one of said chambers and said anode in the other chamber, and said control electrode being in said narrow passage, said control electrode being in the form of a flattened loop of strip material and oriented with the edges of said strip presented to said cathode and anode.

3. A grid controlled gas discharge tube including a thermionic cathode, a control electrode, an anode, and a shield electrode, said shield electrode enclosing a pair of enlarged chambers connected by a narrow passage, said cathode being in one of said chambers and said anode in the other chamber, and said control electrode being in said narrow passage, said control electrode being in the form of a attened loop of strip material having the width 0f said material parallel to the walls of said passage.

4. A grid controlled gas discharge tube including a cathode, a control electrode, and an anode mounted generally along a straight line and a shield electrode enclosing the others of said electrodes, said shield electrode being generally lemniscate in plan with a restricted passage connecting the loops thereof said control electrode being located in said restricted passage.

5. A grid controlled gas discharge tube including a cathode, a control electrode, and an anode mounted generally along a straight line and a shield electrode enclosing the others of said electrodes, said shield electrode being generally lemniscate in plan with a restricted passage connecting the loops thereof, said control electrode being mounted in said passage, said control electrode being in the form of a ilattened loop of strip material and oriented with the edges of said strip presented to said cathode and anode.

6. A grid controlled gas discharge tube including a thermionic cathode, a control electrode, and an anode mounted generally along a straight line, and a shield electrode enclosing the others of said electrodes, said shield electrode being generally lemniscate in plan with a restricted passage connecting the loops thereof, said control electrode being mounted in said restricted passage, said shield electrode having an aperture at one end adjacent said anode and said anode blocking said aperture from view of said cathode.

7. A discharge tube including, within a hermetically sealed casing, a cathode, a control electrode, an anode, and a shield electrode, said shield electrode enclosing a pair of enlarged chambers connected by a narrow passage, said cathode being in one of said chambers and said anode in the other chamber, and said control electrode being in said narrow passage.

8. A discharge tube including a hermetically sealed casing containing an ionizable gas, a cathode, a control electrode, an anode, and a shield electrode Within said casing, said shield electrode enclosing a pair of enlarged chambers connected by a narrow passage, said cathode being in one of said chambers and said anode in the other chamber, and said control electrode being in said narrow passage, said control electrode being in the form of a attened loopof strip material and oriented with the edges of said strip presented to said cathode and anode.

9. A gas discharge tube including a hermetcally sealed casing containing an ionizable gas, a thermionic cathode, a control electrode, an anode, and a shield electrode within said casing, said shield electrode enclosing a pair of enlarged chambers connected by a narrow passage, said cathode being in one of said chambers and said anode in the other chamber, and said control electrode being in said narrow passage, said control electrode being in the form of a attened loop of strip material having the width of said material parallel to the walls of saidpassage.

l0. A grid controlled gas discharge tube incluoling a casing containing an ionizable gas, a thermionic cathode, a control electrode, and an anode mounted generally along a straight line within said casing, and a shield electrode enclosing the others of said electrodes, sa'id shield electrode being generally lemniscate in plan with a restricted passage connecting the loops thereof, said control electrode being mounted in said restricted passage.

11. A grid controlled gas discharge tube including a casing containing an ionizable gas, a thermionic cathode, a control electrode, and an anode mounted generally along a straight line within said casing, and a shield electrode enclosing the others of said electrodes, said shield electrode being generally lemniscate in plan with a restricted passage connecting the loops thereof, said control electrode being mounted in said restricted passage, said shield electrode having an aperture at one end adjacent said anode and said anode blocking said aperture from view of said cathode.

FORREST E. GEHRKE.

REFERENCES CITED UNITED STATES PATENTS Name Date Freeman Nov. 18, 1947 Number 

